feat: 1995

engine/scene/Sprite.ts

@@ -0,0 +1,45 @@
+import type { Vec2 } from "../math/Vec2"
+import type { Vec3 } from "../math/Vec3"
+import type { Texture } from "../render/Texture"
+import { Camera } from "./Camera"
+import type { Mesh } from "./Mesh"
+
+/** A flat image standing in the world, always turned to face the camera --
+ *  how the PS1 drew most enemies and props instead of 3D models. */
+export type Sprite = {
+  /** World anchor at the base (feet) center. */
+  position: Vec3
+  /** World-space width and height. */
+  size: Vec2
+  texture: Texture
+}
+
+export namespace Sprite {
+  /**
+   * Build the sprite's quad as a Y-axis billboard: it spins around vertical to
+   * face the camera but stays upright, so characters never tilt. Draw the
+   * result with Rasterizer.draw (its alpha cutout hides transparent texels).
+   */
+  export function billboard(sprite: Sprite, camera: Camera): Mesh {
+    const forward = Camera.forward(camera)
+    // Camera right projected onto the ground plane (== normalize(-fz, 0, fx)).
+    const len = Math.hypot(forward.x, forward.z) || 1
+    const rx = -forward.z / len
+    const rz = forward.x / len
+    const hw = sprite.size.x / 2
+    const p = sprite.position
+    const y0 = p.y
+    const y1 = p.y + sprite.size.y
+    const lx = p.x - rx * hw
+    const lz = p.z - rz * hw
+    const gx = p.x + rx * hw
+    const gz = p.z + rz * hw
+    const vertices = [
+      { pos: { x: lx, y: y0, z: lz }, uv: { x: 0, y: 1 } },
+      { pos: { x: gx, y: y0, z: gz }, uv: { x: 1, y: 1 } },
+      { pos: { x: gx, y: y1, z: gz }, uv: { x: 1, y: 0 } },
+      { pos: { x: lx, y: y1, z: lz }, uv: { x: 0, y: 0 } },
+    ]
+    return { vertices, indices: [0, 1, 2, 0, 2, 3] }
+  }
+}